1. Field of the Invention
The present invention relates to a projection exposure apparatus (projection printer) in use for LSI fabrication processes.
2. Description of the Related Art
FIG. 6 shows an optical system of a conventional projection printer. A fly-eye lens unit 3 is arranged in front of a lamp 1 and a mirror 2. In front of the fly-eye lens unit 3, an aperture unit 4, condensers 5 and 6, and a mirror 7 are located. A mask 8 including desired circuit patterns is next to the condensing lens 6. A wafer 10 and a projection lens unit 9 are placed in front of the mask 8. Fly-eye lens unit 3 has a fly-eye lens 12 fixed to an outer frame 13. As shown in FIG. 7, the fly-eye lens 12 comprises a plurality of identical lenses 11 having a rectangular shape in a plan view which are arranged in a two-dimensional array. The line on the fly-eye lens unit 3 designated by alternating long and short dashes corresponds to the edge of an aperture 4a of the aperture unit 4. In FIG. 6, the outer frame 13 of the fly-eye lens unit 3 is not shown and the number of element lenses 11 is reduced for simplicity.
Light emitted from the lamp 1 is introduced to the fly-eye lens unit 3 via the mirror 2. Light which is incident into each element lens 11 of the fly-eye lens unit 3 illuminates all of the mask 8 via the aperture 4a of the aperture unit 4, the condenser 5, the mirror 7, and condenser 6. In FIG. 6, there are shown only light beams which pass through element lenses 11 located at the central part of the fly-eye lens unit 3 to illuminate the mask 8, but light beams passing through the other element lenses 11 also illuminate the mask 8 in similar way so that the whole exposure area is illuminated. As a result, the light beams from each element lens 11 of the fly-eye lens unit overlap on the surface of the mask 8; thus, uniform illumination is achieved. Furthermore, after passing through the mask 8, the light reaches the wafer 10 via the projection lens unit 9 to transfer the circuit patterns onto the surface of the wafer 10.
In such a projection printer, it is known that the illumination with shorter wavelength light results in better resolution suitable for exposure of finer patterns. Therefore, use of an excimer laser producing far ultraviolet wavelength light has been investigated.
However, the laser beam emitted from an excimer laser exhibits high coherence, hence interference patterns appear on the surface of mask 8 also on the surface of wafer 10 when it is used as a light source in the projection printer of FIG. 6. Such irregularity in the illumination is a problem to be solved.